A farmer discovers a natural gas reserve on his property. He can extract the natural gas for a profit of $20 per unit now, $25 per unit in one year, $30 per unit in two years, and $35 in three years....

A farmer discovers a natural gas reserve on his property. He can extract the natural gas for a profit of $20 per unit now, $25 per unit in one year, $30 per unit in two years, and $35 in three years. The current market rate of interest is 6 percent. When should the farmer extract the natural gas to obtain the most profit per unit in present value terms?